1. Field of the Invention
The present invention relates generally to a shift control apparatus for an automatic transmission of a motor vehicle, and more particularly to an arrangement for suitably determining a compensating coefficient used in determining the need for shifting of the transmission according to a predetermined shift pattern.
2. Discussion of the Prior Art
For controlling an automatic transmission of a motor vehicle, there is commonly used a shift control apparatus which is equipped with shift control means for automatically shifting the automatic transmission according to a predetermined shift pattern and on the basis of actually detected values of operating parameters of the vehicle. The shift pattern represents a relationship of the operating parameters. FIGS. 4(a), 4(b) and 4(c) show examples of shift patterns in the form of shift-up boundary lines used for determining the necessity of shifting up an automatic transmission having four forward drive positions, i.e., 1st-speed position, 2nd-speed position, 3rd-speed position and 4th-speed position. FIGS. 5(a), 5(b) and 5(c) show examples of shift patterns in the form of shift-down boundary lines for determining the necessity of shifting down the transmission. Each shift-up or shift-down boundary line represents a relationship between two operating parameters of the vehicle, that is, between the opening TA of an engine throttle valve and the running speed V of the vehicle. The determination as to whether the transmission is shifted up or down from the current position is effected according to the shift-up and shift-doom boundary lines for the current position, and depending upon the currently detected vehicle speed V and throttle opening TA.
The throttle opening is used as a parameter which represents the currently required output of the engine, or the load currently applied to the engine. In recent motor vehicles, however, the throttle opening does not necessarily represents the currently required engine output with high precision, since the engines of the recent vehicles are equipped with various mechanisms or devices for improving the fuel economy of the vehicle and suitably controlling the output of the engine depending upon the running condition of the vehicles. For instance, there has been proposed an engine equipped with a valve timing changing device for changing the opening and closing timings of the intake and exhaust valves, and/or an idling control valve for adjusting the idling speed of the engine. The throttle opening does not precisely represent the currently required output of the engine, since the actual intake air quantity of the engine varies depending upon the altitude, namely, varies with the atmospheric pressure. In view of these phenomena, there is proposed a shift control apparatus which includes (a) shift control means as described above, (b) detecting means for detecting the engine speed NE of the engine, the throttle opening TA and an intake air quantity Qm of the engine, (c) coefficient calculating means for calculating a currently required intake air quantity Qc of the engine from the detected engine speed NE and throttle opening TA, and calculating a compensating coefficient K by dividing the calculated required intake air quantity Qc by the detected intake air quantity Qm, and (d) compensating means for changing or compensating one of the shift pattern (shift boundary line) and the actually detected values of the operating parameters. An example of such shift control apparatus is disclosed in laid-open publication No. 2-266155 (published in 1990). In this shift control apparatus, the required intake air quantity Qc is obtained according to a predetermined relationship between the engine speed NE and the throttle opening TA, and on the basis of the actually detected values of these parameters NE, TA. Then, the compensating coefficient K=Qc/Qm is calculated by dividing the calculated required intake air quantity Qc by the intake air quantity Qm actually measured by an air flow meter. The actually detected throttle opening TA is then compensated by multiplying the detected value TA by the calculated compensating coefficient. The determination as to whether the transmission is shifted up or down is effected according to the shift pattern, and on the basis of the compensated throttle opening TA, and the vehicle speed V as detected by the detecting means. Alternatively, the determination is effected according to one of shift boundary lines which is selected depending upon the calculated compensating coefficient K. In the latter case, the determination is based on the throttle opening TA and vehicle speed V which are detected by the detecting means.
A study of the present applicants revealed a problem with the known shift control apparatus as described above, which arises upon a sudden or abrupt change in the throttle opening TA as indicated in FIG. 7(d). More specifically, the change in the actual intake air quantity Qm upon such sudden change in the throttle opening TA is delayed with respect to the change in the calculated required intake air quantity Qc, as indicated in two-dot chain line in FIG. 7(c). As a result, the calculated compensating coefficient K which is equal to Qc/Qm suffers from a sudden change, as indicated in one-dot chain line in FIG. 7(b), causing busy shift-down and shift-up actions in a short time following the sudden change in the throttle opening TA, as indicated in one-dot chain line in FIG. 7(a). In the example of FIGS. 7(a)-7(d), the transmission is shifted first from the 4th-speed position to the 3rd-speed position, then to the 2nd-speed position, and is subsequently shifted up to the 3rd-speed position and finally to the 4th-speed position.
The known shift control apparatus also suffers from a problem that the transmission is not shifted down even when the vehicle driver desires to shift down the transmission by depressing the accelerator pedal. In other words, the known shift control apparatus is not arranged so as to reflect the driver's desire to accelerate the vehicle. In this respect, it is noted that the transmission is likely to be shifted down upon depression of the accelerator pedal, since the compensating coefficient K increases due to a delay in the change of the actual intake air quantity Qm with respect to the change of the throttle opening TA, as described above. However, the transmission is shifted up a short time after the shift-down action. This is contrary to the driver's desire to accelerate the vehicle when the driver depresses the accelerator pedal.
In the shift control apparatus wherein the shift pattern or the appropriate operating parameter is compensated by the coefficient K (=Qc/Qm) as described above, some errors are inevitably included in the detected values of the engine speed NE, throttle opening TA and intake air quantity Qm, and in the calculated value of the required intake air quantity Qc obtained from the detected engine speed NE and throttle opening TA. In particular, the errors are unavoidably present in the calculated required intake air quantity Qc and the detected actual intake air quantity Qm, when these values Qc and Qm are relatively low. Even if the amounts of these errors are small, the errors will have a considerable influence on the calculated compensating coefficient K, and the compensation using the compensating coefficient will deteriorate the manner of shifting of the transmission. This is a third problem to be solved by the present invention.
The conventional shift control apparatus also suffers from a problem when an air conditioner is in operation. More specifically, when the air conditioner is driven by the engine, the intake air quantity is increased by a suitable amount corresponding to an amount of power consumption by the air conditioner. This increase in the intake air quantity Qm is effected by the idling control valve which opens an appropriate air by-pass passage which by-passes the throttle valve, so that the idling speed of the engine is increased, irrespective of the opening of the throttle valve. Since the compensating coefficient K (=Qc/Qm) is determined by the actual intake air quantity Qm, the coefficient K decreases when the intake air quantity Qm increases when the idling control valve is activated, namely, when the air conditioner is on. When the vehicle is accelerating, for example, the decrease in the compensating coefficient K results in lowering the shift-up vehicle speed, whereby the transmission is likely to be shifted up at a relatively early point of time. Thus, the activation of the air conditioner lowers accelerating capability or drivability of the vehicle, due to a decrease in the compensating coefficient K caused by the increased intake air quantity Qm.
The conventional shift control apparatus does not have a fail-safe function for suitably controlling the transmission in the even of failure of any of the detectors for sensing the intake air quantity, revolution speed and throttle opening of the engine, which are used to calculate the compensating coefficient K (Qc/Qm). If any of such detectors happened to be defective or fail to normally function, by any chance, the calculated compensating coefficient K would deviate from a normal range, and the shift control means would operate according to the improperly compensated shift pattern or operating parameter, whereby the transmission might exhibit an inadequate shift-up and shift-down behavior, leading to unsatisfactory running performance of the vehicle such as low fuel economy and insufficient accelerating capability.
The conventional shift control apparatus also suffers from a drawback due to chronological deterioration or change of the engine. Described in detail, the output torque characteristics of the engine will change due to an increase in the friction loss and the pumping loss during use. To maintain the nominal engine output torque, the intake air quantity Qm is increased by the idling control valve and valve timing changing device, for example, as the friction and pumping losses increase during use of the engine. Thus, the intake air quantity Qm required to provide the nominal engine torque increases as the service period of the engine increases. On the other hand, a stored data map representative of a predetermined relationship for calculating the required intake air quantity Qc is formulated based on the initial output torque characteristics of the engine. For these reasons, the compensating coefficient K obtained from the detected actual intake air quantity Qm and the calculated required intake air quantity Qc varies with the chronological deterioration or change of the engine.